Thinking about it after reading the other answers, I feel the only real pragmatic answer is a combination of ignorance and a superhuman amount of resilience.
You could have a shit ton of money, but society could go belly-up and suddenly all those pieces of paper/plastic or numbers on a bank app screen don't mean jack. You could have whatever is plaguing you at the moment instantaneously cured, but some other horrible catastrophe of another nature you're not equipped to deal with could come out of nowhere and leave you flat on your ass. You could have whatever one single thing you feel you need to make life worth it now, but you could either lose it or the stability it provides, or something else that suddenly makes that stability insufficient. And most likely, given the way things happen, that would just be a matter of time.
If I was resilient beyond reason though, it wouldn't matter. I could deal with my situation as it is without being so existentially fucking miserable. And if I was ignorant about the awfully cruel apparent nature of reality, I'd still be chipper if things got better. Or even if they got worse - suddenly broke, on the street, in far worse physical and mental shape than I am now, maybe without the basic requisites for life...and I'd be fine with it. Not to say I'd accept it, but I wouldn't be thinking of suicide as a solution. Same goes for if I suddenly had among the best circumstances among humans. No blow would make me falter, neither real nor imagined.
This, I think, is the key to passion for life. Sadly it's a key I think I will never find.